Power utilities use a variety of technologies to reduce the impact of disturbances in the power grid to lower the risk of blackouts. Many of these are commonly referred to as Flexible AC Transmission System (FACTS) devices. A well known FACTS device is the controlled series compensator, CSC. A CSC commonly comprises a series connected reactance with a parallel branch containing a switch. The reactance may either be a capacitance or an inductance or a combination of the two. The switch may either be a mechanical switch or a semiconductor switch. Two well known such FACTS devices are the Thyristor Controlled Series Compensator (TCSC) and the Thyristor Switched Series Compensator (TSSC). A CSC is based on the principle of varying the power line series reactance in order to control power flows and enhance system stability. The most important phenomena which affect the stability of power systems are poorly damped low frequency electro-mechanical oscillations, first-swing instabilities and voltage instabilities.
Power oscillation damping is traditionally improved by the use of Power System Stabilizers (PSS) that act on the Automatic Voltage Regulators (AVR) which control the generators in the power system. The structure of the power system determines the effectiveness of the PSS. In some cases the damping of inter-area modes may be inadequate. In these cases supplementary damping may be added to the power system by installation of a FACTS device like the TCSC at a proper location. However, the design of an effective controller for such a damping device is complicated. The equations governing the oscillations in a power system are non-linear since the power flow on one transmission (in per unit) comprises a sine function of the voltage phase angle difference at the line ends; divided by the line reactance (in p.u). Further the power system parameters often change dramatically during the contingencies causing the power oscillation. Consequently a controller which offers a good performance in one mode of operating and one system configuration may be inadequate in another mode of operation or a second system configuration. This may result in a negative damping of the power oscillation, which ends up in a power system failure. Therefore the design of a damping controller must take into account several operating conditions as well as system configurations thus making it hard to find the optimal design.
From U.S. Pat. No. 6,559,561 (Angquist) a method and device for damping power oscillations in transmission lines is previously known. The object of the method is to provide fast and robust identification of a component of the power oscillation. Accordingly a priori knowledge of the expected frequency of the power oscillation is utilized for estimating the oscillating component of the oscillation without any annoying influence of the simultaneous mean-power change and oscillations with deviating frequency. Depending on what kind of actuator that is utilized for the damping, an additional adaptation of the damping signal may be carried out. This applies, for example, when the actuator is in the form of a controllable series capacitor which is controlled with a reference value for its reactance, or in the form of a static reactive-power compensator which is controlled with a reference value for its sensitiveness.
Power systems with inherent inter-area oscillation modes are generally vulnerable to transient instability or to first-swing instability. This phenomenon may arise when a fault leads to an interrupted power transmission between a sending power grid area and a receiving power grid area. This leads to an advance in the generator phase angles in the sending area and a retardation of the phase angles for the machines in the receiving area. Once the fault has been cleared, the resulting difference in speed of the machines in the sending area and in the receiving area may lead to a loss of synchronism. The transfer capacity of the interconnection lines between the two areas is then insufficient after the fault. This type of event is in a first order approximation governed by a so called equal-area criterion. When the allowed maximum power transfer of an interconnecting line in a system is determined a high level of power transfer leads to a lower margin to transient instability.